1. Field of the Invention
This invention relates to the separation of the components of a gas mixture by adsorption and more particularly to the high purity recovery of the more readily adsorbed gas component and the less readily adsorbed component products of a pressure swing adsorption (PSA) process.
2. Description of the Related Art
There is often a need to recover the constituents of a gas mixture in high purities. For example, when a flammable gas, such as methane, is to be recovered from a gas stream which contains methane and oxygen, either in the presence or absence of other gaseous constituents, it is often desirable and usually necessary to separate the mixture into a nonflammable methane-rich component and a nonflammable methane-lean component. The nonflammable methane-rich component is a mixture of methane and oxygen which is too rich in methane and too lean in oxygen to constitute a flammable mixture and the nonflammable methane-lean component is a mixture of methane and oxygen which is too lean in methane and too rich in oxygen to constitute a flammable mixture.
Potentially flammable gas mixtures are formed in nature or are produced in industrial processes. For example flammable gas mixtures are formed by the release of methane from landfills or from coal when the released methane combines with air to produce a mixture containing about 6% to 14% by volume methane. Flammable gas mixtures are produced in industry by the release of methane from coal during mining or by the release into the air of various other hydrocarbons, such as ethylene, butene, etc. These compounds are used in various chemical reactions, such as polymerization.
A number of methods are available for separating the constituents of the above-described gas mixtures. A highly effective method for separating the constituents of gaseous mixtures is cryogenic distillation. This technique is very useful for large volume separation plants in which the feed gas being treated does not contain mixtures of oxygen and flammable gases. However, if a mixture of a flammable gas and an oxygen-containing gas, such as air, is cryogenically separated in a distillation column into substantially pure flammable gas and oxygen streams, a flammable mixture will be formed at some point within the column, thereby creating an explosion hazard. Accordingly, cryogenic separation cannot be safely used to separate mixtures of oxygen and flammable gases.
Cryogenic separation can be used to separate the flammable gas from the other constituents of a gaseous mixture which also contains oxygen, if the oxygen is first removed from the gas mixture. For example, the oxygen can be removed from the mixture by catalytic combustion of the oxygen and a portion of the flammable gas. This method of separation likewise has major drawbacks. Firstly, the oxidation is highly exothermic, and provision must be made for waste heat removal. Secondly, some of the flammable gas is used up in the combustion. It can readily be appreciated, therefore, that this method of flammable gas recovery can only be used when oxygen is present in the gas mixture in very small concentrations, such as, for example, concentrations of less than one volume percent.
Another procedure that has been considered for recovering flammable gases from oxygen-containing gas mixtures is adsorption. According to this procedure, the gas mixture is passed through an adsorbent which adsorbs the flammable gas in preference to the other components of the mixture. The flammable gas is generally recovered as the desorbed product stream and the other gases in the mixture are generally recovered as the nonadsorbed product stream. However, conventional adsorption procedures are not practical for the recovery of flammable gas from oxygen-flammable gas mixtures because when the process is optimized to produce high purity flammable gas, it is difficult to produce a nonadsorbed gas stream which contains the flammable gas at a concentration less than the lower flammable mixture limit.
U.S. Pat. No. 4,305,734, issued to McGill, discloses a process for separating flammable gases such as methane from flammable gas-air mixtures by passing the gas mixture through a bed of adsorbent which adsorbs the flammable gas in preference to air. The principal purification step of the disclosed Process comprises passing the feed mixture through the adsorption bed while simultaneously producing non-adsorbed gas from the adsorber. This step is followed by cocurrently flushing carrier gas from the adsorber with flammable gas and then recovering flammable gas from the adsorber by depressurization of the adsorber. Although this process may result in increased feed gas throughput, the purity of the desorbed product can be lower than is desired since the inlet region of the adsorber will contain a greater concentration of carrier gas when the flammable gas purge step begins.
U.S. Pat. No. 3,797,201, issued to Tamura, discloses the production of high purity desorbed and nonadsorbed gaseous products by a process which includes a step in which desorbed product is used to cocurrently purge the adsorber at the pressure at which the adsorption step is carried out and a step in which nonadsorbed product is produced while fresh feed is introduced into the adsorber.
U.S. Pat. No. 4,070,164, issued to Miwa et al., discloses the adsorptive separation of the components of a two gas component mixture. The process comprises four serially conducted steps, including feed gas pressurization without removal of nonadsorbed gas, cocurrent purge with desorbed product while nonadsorbed product is cocurrently removed from the adsorber, a first countercurrent depressurization and a second countercurrent depressurization.
U.S. Pat. No. 4,813,980, issued to Sircar, discloses a PSA process for the separation of a mixture of hydrogen and nitrogen from carbon dioxide. The process of this patent includes a high pressure co-current rinse of the bed with desorbed gas following the adsorption step.
U.S. Pat. No. 4,013,429, issued to Sircar et al., discloses the separation of nitrogen from air by a PSA process in which the bed is rinsed with desorbed nitrogen following the adsorption step. The effluent obtained during the rinse step may be added to the fresh air feed.
U.S. Pat. No. 4,963,339, issued to Krishnamurthy et al., discloses the production of high purity hydrogen and carbon dioxide from a gas mixture by a PSA process which includes the step of cocurrently purging the bed with desorbed product following a depressurization of the bed into an equalization tank.
Because of the desirable features of adsorption there is a need for an adsorption process which can simultaneously effect the efficient separation of gas mixtures into a high purity desorbed product stream and a nonadsorbed product stream which contains very little strongly adsorbed constituent. The present invention satisfies that need.